Frequency-dependent relaxation rate in superconducting YBa2Cu3O6+delta

The submillimeter-wave 3 cm(-1) < v < 40 cm(-1) complex conductivity of the reduced YBa2Cu3O6+delta film (T-C = 56.5 K) was investigated for temperatu res 4 K < T < 300 K and compared to the properties of the same him in the o ptimally doped state. The frequency dependence of the effective quasipartic le scattering rate 1/tau* (v) was extracted from the spectra. 1/tau* is sho wn to be frequency independent at low frequencies and high temperatures. A gradual change to 1/tau*alpha v(1.75+/-0.3) law is observed as temperature decreases. In order to explain the observed temperature dependence of the l ow frequency spectral weight above T,, the quasiparticle effective mass is supposed to be temperature dependent for T > T-c.